It's a "musical shooter". You have to play the music (something like Donkey Konga for NGC) and kill the enemies. Press the key corresponding to the pad on the bottom screen when it passes through the white circle and you wil lshoot . the more accurate is your press, better the shoot and faster the bar for the special shoot will fill up.
controls:
A / B / X / Y: play the music and shoot.
L: shoots the weakest laser.
R: special shoot (when special bar is filled)
L+R+Select+Start: reset the game.
Sometimes the sprite system might look weird (hanging shoots, sprites looking terrible, etc.), but it's a bug I didn't have the proper time to fix (if you don't shoot A LOT it doesn't seem to happen, though). When it happens, just reset the game (L+R+Select+Start) or keep playing it until it's over, it just looks awfull, but nothing that will interfere on the engine system, so it's still playable